Be careful with those boxes. They are creative marketing at work. While they are technically attenuators, they are NOT to be confused with speaker attenuators, power soaks, dummy loads, etc. NEVER put one of these boxes between your amp and speaker, or else you can seriously damage your amp. These ebay boxes are nothing more than a simple, cheap volume control that's put in the FX loop. If your amp has a master volume, these boxes do exactly the same thing as turning down the master volume on your amp. This is not the kind of attenuation that most players are looking for. Most want to push the power section of the amp harder. The only way to do that is to crank it up and put a proper, impedance matched attenuator with adequate power handling capabilites between the amp and the speaker. Now, if you have a non-master volume amp with an effects loop, these boxes would allow you to add a master volume in the effects loop. However, this would NOT push the power section any harder, and any extra distortion you would get would be preamp distortion. This may or may not be what you want.
As far as "real" attenuators, check out the Weber Mass, THD Hotplate, Dr. Z Airbrake, and Marshall Power Brake. Those are designed to keep the proper load on the power amp and dissipate the extra power that would be going to the speakers. I don't know of any kits out there, but Ted Weber does sell the speaker motors that are used in the Weber Mass. If you go this route, just make sure you know what you are doing. An improper load on a tube amp can cause serious and expensive damage. Good luck.
